1. Field of the Invention
The present invention relates to an apparatus and method for processing an edge of a glass sheet. More particularly, the present invention relates to an apparatus and method for grinding or polishing an edge of a glass sheet that can be used in a flat panel display.
2. Technical Background
Liquid crystal displays (LCDs) are passive flat panel displays which depend upon external sources of light for illumination. They are manufactured as segmented displays or in one of two basic configurations. The substrate needs (other than being transparent and capable of withstanding the chemical conditions to which it is exposed during display processing) of the two matrix types vary. The first type is intrinsic matrix addressed, relying upon the threshold properties of the liquid crystal material. The second is extrinsic matrix or active matrix (AM) addressed, in which an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. In both cases, two sheets of glass form the structure of the display. The separation between the two sheets is the critical gap dimension, of the order of 5-10 μm. The individual glass substrate sheets are typically less than about 0.7 mm in thickness.
Processing glass sheets that require a high quality surface finish like the ones used in flat panel displays typically involves cutting the glass sheet into a desired shape and then grinding and/or polishing the edges of the cut glass sheet to remove any sharp corners. Grinding and polishing steps may, for example, be carried out on an apparatus known as a double edger or double edging machine. Such double edging machines are known and available from Bando Kiko Co., Ltd., Mitsubishi Heavy Industries, Fukuyama Co., and Glass Machinery Engineering.
During the grinding and polishing of the edges of a glass sheet using a double edging machine, the glass sheet is typically sandwiched between two neoprene or rubber belts. The belts contact both surfaces of the glass sheet and cooperate to hold the glass sheet in place while the edges of the glass sheet are ground or polished by an abrasive grinding wheel. The belts also transport the glass sheet through a feeding section of the machine, a grinding or polishing section of the machine, and an end section of the machine.
This method of gripping, processing and conveying a glass sheet using a double edging machine has several disadvantages. First, the particles generated during edge finishing can be a major source of contamination on the surfaces of the glass sheet. Thus, the glass sheet requires extensive washing and drying at the end of the finishing process to clean and wash off the generated particles. Of course, the additional steps of washing and drying at the end of the finishing process impacts the original cost for the finishing line and increases the cost of manufacturing. Secondly, the particles and chips caught between the belts and the glass sheet can severely damage the surfaces of the glass sheet. Sometimes this damage can be the cause of a break source during subsequent processing steps and result in poor process yields due to a reduced number of selects that can be shipped to a customer.
To address the cleanliness concerns, the surfaces of the glass sheet can be protected by a plastic film to help prevent damage and contamination. But, if the source of contamination can be eliminated or minimized, then the plastic film is not needed and that would reduce the cost and complexity of the finishing process.
U.S. Patent Application US2005/0090189 describes a process and apparatus for grinding and/or polishing the edge of a glass sheet wherein pressurized air is distributed through opposing porous plates to prevent particulate generated by the edge processing from contaminating the glass sheet, thereby eliminating the need for plastic coatings. In spite of this advance however, porous plates are subject to low air flow, limiting the effectiveness of the plates in preventing particulate contamination. Moreover, to obtain an effective seal at low air flow rates, the plates must be relatively wide, thus increasing the amount of glass overhanging the glass support and maintained between the plates. Another drawback is that the air flow coming out of the porous plates is non-directional, which decreases its effectiveness in containing the grinding particles and coolant.
For thin glass sheets, such as are used in LCD display devices, vibration due to excess overhang can result in unacceptably rough processed edges.
Accordingly, there is a need for an apparatus and method that helps prevent particles and other contaminants that are generated during edge finishing from contaminating or damaging the two surfaces of a glass sheet, while providing for clean, chip-free processed edges. Moreover, minimizing the generated particle levels would reduce the load on the washing equipment downstream.